The problem as I understand it is that if both fans are using the same duct and are both running at the same time, the fans will be less efficient and more noisy because it will be like installing a fan with a smaller duct that it was rated for. The fan(s) would have to work harder and would expel less air. I doubt any inspector would pass such an install. But this is just my opinion since I was looking to do this kind of setup until I realized it was not worth it.

Fantec makes some nice multiple input fan systems. You can turn it on or off from either location. As Terry mentioned, these use a remote fan and are very quiet, so much so that you may want to put a pilot light to indicate that they are on.

i think it may be possible to arrange for one fan to suck air equally from two inlets. So both are ON or both are OFF. Not one or the other.

a month ago i looked on the Broan and Nutone web sites (they are sister companies), and I saw a remote-installed squirrel cage fan that came with two vent inlets attached to a splitter-combiner, so when it was ON, it pulled from both inlets. Hopefully equally, but that would depend on the length and elbows after the combiner-splitter.

I have a squirrel cage installed remotely. The noise is nice because it is only the noise of air moving through the vent opening, not a motor noise. I am planning on putting a variable speed controller on it because it is too powerful by a factor or two or three, and I would like to be able to leave it ON, on "low", for hours and hours, when the outdoor temperature is just right.